High-temperature-strength precipitation-hardenable austenitic iron-base alloys

ABSTRACT

This invention is for an iron-base alloy system characterized in that (a) it consists of an iron-nickel-chromium matrix which maintains substantially complete austenitic stability at temperatures from room temperature to the melting point; (b) it is susceptible to treatment to effect a precipitation-hardening resulting in a molybdenum-containing precipitated phase; (c) said alloy consists essentially of, in atom percent, 20 to 30 percent nickel, 5 to 10 percent chromium, a strength-inducing additive selected from either molybdenum or tungsten in an amount not exceeding 10 percent, and the balance iron.

mted States Patent [151 3,640,704 Robertshaw et al. Feb. 8, 1972 [54] HIGH-TEMPERATURE-STRENGTH, [S6] Reterenoes Cited PRECIPITATION-HARDENABLE, UNITED STATES PATENTS AUSTENlTIC IRON-BASE ALLOYS 2,725,493 11/1955 Mitchel ..75/ I28 R [72] Inventors: Fred C. Robertshaw, Cincinnati; Jon L. 3,362'855 1/ 1968 I R Bartos, Loveland; James J Hurst Clark 3,547,625 l2/I970 Bieber ..75/I28 R ll ames August McGurt C ti, g mcmna Primary Examiner-HyIand Bizot Attorney-Roland A. Anderson [73] Assignee: The United States of America as represented by the United States Atomic [57] ABSTRACT Energy Co This invention is for an iron-base alloy system characterized in [22] Filed: Jan. 20, 1970 that (a) it consists of an iron-nickel-chromium matrix which maintains substantially complete austenitic stability at tem- [21 1 Appl' peratures from room temperature to the melting point; (b) it is susceptible to treatment to effect a precipitation-hardening 52] us. Cl. ..7s/12s w, 75/128 R resulting in molybdenum-(mining Precipitated Phase; (c) 51 Int. Cl C22c 39/20 Said miss mentia'ly in 3mm Percent [58] Field of Search ..75/l28 nickel 5 Permt chromium a srength'inducing I additive selected from either molybdenum or tungsten in an amount not exceeding I0 percent, and the balance iron.

3 Claims, 2 Drawing Figures 72 w l e /o 3: f a 64 o c o m.

7 A I A l I 56 SOLUTION TREATMENT, 12o5c 1 HOUR, WATER OUENCHED, FOLLOWED BY :1 PRECIPITATION HARDENING OF n 48 Fe-ZO Ni-SCr-IOMO g 7 D o 550 c g 0 J 0 650 c m 8 o 750C o a a5oc X SOLUTION HARDENED 32 TIME, HOURS wGDOI wzz. 00m

7 INVENTORS. Fred C. Roberfshaw James A. MEGurfy James J. Hurst Jon L. Bartos ATTORNEY.

SHEET 1 BF 2 0000 0 000mm 0 000mm 10 PAIENTED E 8 I972 assanouvu STRESS, ksi

PATENTEnrza 8 r912 SHEET 2 OF 2 PARAMETRIC REPRESENTATION OF THE CREEP RUPTURE LIFE 0 0F Fe-2ONi-5Cr-10Mo and 31s STN.STL. 21

in 0 X N E a U (D ll] 05 k U) nus-71 so| (131sc) AGE (now/50m) o 316 STN.STL.ANNEALED (112cm) so 32 34 as as. 40

P=(T+460) (18+Log n 1o' Fig.2

INVENTORS.

Fred C. Robertshaw James A..M Gurfy James J. Hurst y Jon L. Barfos ATTORNEY.

HlGH-TEMPERATURE-STRENGTH, PRECIPITATION- HARDENABLE, AUSTENITIC, IRON-BASE ALLOYS BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The present invention relates to a selected class of precipitation-hardenable iron-base alloys containing nickel, chromium, molybdenum, and tungsten in specifically defined ranges of concentration toensure a stabilized austenitic (i.e., face-centered cubic or gamma) crystallographic phase over the full range of temperature from C. up to the melting point. While the iron-base alloys of this invention are not steels, they do contain carbon in extremely low quantifies, in the parts-per-million range, which do not critically determine anyphase relationship or physical property of the hereindescribed alloy system. i m

It is a general object of this invention to provide an ironnickel-chromium-molybdenum or tungsten-containing alloy system capable of being hardened by a precipitation heat treatment to achievea practical combination of hardness, ductility, short-term and long-term (creep) strength under sustained operation at low and particularly at high (550-750 C.) service temperatures. A specific object is to provide a stabilized iron-base face-centered cubic alloy system with a creep strength at a temperature in the range 550 to 750 C. which is at least equal to or superior to austenitic stainless steels having useful properties in the same range of temperature.

SUMMARY OF THE INVENTION The inventive concept herein disclosed is based on a process of judicious selection of alloy components which enable the definition of an alloy system characterized by a stabilized austenitic crystallography over the temperature range 0 to 1,400 C. This means that the maximum physical properties of the alloy can be utilized without experiencing the changes or discontinuities normally accompanying crystallographic phase transformations in iron-base alloy systems. In addition, the alloy system is precipitation-hardenable to a microstructure which is stable over long periods of time at elevated temperatures without overaging to a brittle condition. The alloy is easily fabricable by standard forming techniques into a variety of forms and shapes; for example, into sheet or tube form. Such an alloy system, in accordance with our invention, consists essentially of, in atom percent, 20-30 percent nickel, 5-10 percent chromium, 5-10 percent of a strength-inducing element selected from the group molybdenum or tungsten, and the balance iron. The phrase consisting essentially of" is used in this specification to denote positive recitation of essential alloying ingredients but is not meant to rule out the presence of residual amounts of such elements as, by way of example, carbon, boron, and oxygen which are not deliberately added to the alloy for any specific alloying purpose but which occur in low concentrations of the order of from less than 5 to no more than 100 parts per million as impurities which accompany the alloy fabrication process.

The principal criteria for selecting the alloy compositional limits are based on the multifold object of arriving at stable crystallography and microstructure over a wide range of temperature and particularly over the temperature range 550-75 0 C. with satisfactorily high short-term and long-term (creep) properties in a nonoxidizing and/or corrosive atmosphere such as liquid sodium where a satisfactory level is measured against the properties of Type 316 stainless steel (16-18 weight percent chromium, -14 weight percent nickel, 2-3 weight percent molybdenum, 1 (max.) weight percent silicon, 0.03-0.08 weight percent carbon, plus phosphorus and sulfur).

At the chromium and molybdenum levels of interest here, maximum austenite stability is achieved at nickel concentrations in the range 20-30 percent. satisfactorily high shortterm and long-term strength is attained throughout this nickel range but, where the alloy is exposed to liquid sodium, op-

timum sodium corrosion resistance is obtained with nickel concentrations in the range 20-25 percent.

Molybdenum or tungsten is used in the alloy at concentrations which exceed its room temperature solubility, requiring a minimum of about 4 percent of either metal. The presence of molybdenum or tungsten at these concentrations permits attainment of a uniform precipitation-hardened dispersed phase within an iron-nickel-chromium matrix produced by suitable solid solutioning followed by subsequent heat treatment at a lower temperature at least equal to or greater than the intended service temperature to develop and sustain a maximum hardness while avoiding the formation of brittle phases. Concentrations of molybdenum, tungsten alone, or the combination of both which exceed 10 atom percent of the total alloy are undesirable because of degradation of ductility due to the formation of a brittle sigma phase.

The presence of chromium in the range of 5 to 10 percent at the prescribed nickel level with a molybdenum and/or tungsten concentration in the range of about 5 to 10 percent has been found to enhance the stability of the austenite phase. in addition, the combination of chromium and the strength-inducing additive appears to enhance high-temperature strength, especially at temperatures in the range 550-750 C. Furthermore, the essential absence of carbon avoids the common problem experienced with austenitic chromium-nickelcontaining steels of intergranular corrosion due to the formation of precipitated chromium carbides which rob the grain boundaries of the protective effect of chromium.

The phase stability effect of chromium becomes less apparent at concentrations less than 5 percent unless greater amounts of nickel are used within the nickel limits previously described. Thus we have found that a ternary iron-base alloy containing 20 percent nickel, 5 percent molybdenum, and no chromium is unstable and transforms to the body-centered cubic phase, whereas increasing amounts of chromium stabilize the face-centered cubic condition and a S-percent-chromium addition guarantees stability.

The alloys falling within the scope of this invention are easily fabricable by standard metallurgical techniques such as casting or powder metallurgy. We have found that large pure castings are easily made by vacuum induction melting using a zirconia or alumina crucible. For example, in excess of 3-kilogram vacuum-induction-melted castings have been produced which uniformly contain less than about 15 parts per million carbon, less than 10 parts per million nitrogen, and less than 200 parts per million oxygen, as well as less than 5 parts per million boron, with trace amounts of residual elements such 1 as silicon, sulfur, and titanium, the exact amount depending. apparently, upon the composition of the crucible. ln any event, the resultant ingot can be extruded .at a temperature ranging from 1,100 to l,200 C. to an approximately 2- centimeter-diameter rod which can be subsequently forged at the same temperature range to a lesser thickness and then rolled at 1,150 C. to a 0.2S4-centimeter-thick sheet which can be subsequently cold-rolled or hot-rolled to 0.5- centimeter-thick sheet. Cold-rolling to the required thickness is facilitated by several immediate anneals at tempera- The following description provides a representative embodiment of the invention, given in terms of experimental data which illustrate characteristic features of the claimed alloy system of phase stability and high-temperature physical properties as well as illustrating the advantages achieved by precipitation-hardening in improving high-temperature physical properties. The data given in the description are for a preferred alloy of the claimed alloy system which contains 20 percent nickel, 5 percent chromium, and 10 percent molybdenum and represent an excellent combination of phase stability, uniform microstructure, and high-temperature shortterm tensile as well as high-temperature (creep) properties.

Phase Stability A vacuum induction melt made from an alloy consisting of 20 percent nickel, 5 percent chromium. 10 percent molybnecessary to produce a maximum hardening at aging temperatures from 550 to 850 C. It should, however, be recognized that other combinations of heat treatment may also produce a desirable optimum hardness resulting from precipitation-inducing heat treatments.

TABLE II.-EFFECT OF HEAT TREATMENT ON THE TENSILE PROPERTIES OF Fe-20N i-50r-10Mo Test Yield strength Tensile strength Elongatemp tion, Alloy heat No. v Cpndltron C K.s 1 kg./cm. K.s i Kg./cm. percent MS-70 b Sol. 1,205 C./1 hr. water RT 64. 6 4,520 140. 6 9, 900 3. G quenched, then aged 550 52. 1 3, 650 111. 1 7, 880 8. 2 750 C.50 hours. 650 44. 4 3, 100 82. i 5, 800 27. l) 750 37. 6 2, 640 50. 6 3, 540 45. ll

MS-70 Finished-rolled at 1,150 RT 63. 7 4, 450 113. 8 7, 930 18. 5 C. and aged 750 C.50 I 550 53.1 3, 730 88. 2 6, 200 15. 7 hrs. 650 47. 3 3, 310 68. 7 4, 800 31). 4 750 37. J 2, 650 47. J 3, 350 40. 6

Type 31658 Annealed 1,120 C. RT 35. 7 2, 500 S3. 2 5, 820 72. 5 min. 550 16. 0 1, 120 64. 0 4, 580 42. 5 650 13. 8 970 47. 9 3, 350 47. 1 750 13. 0 010 34. 0 2, 380 52. 2

TABLE I Linear Expansion Temperature, A L

' C. L X 100 These results, when plotted on a temperature-versus-percent linear expansion curve, reveal a smooth linear expansion from room temperature to l,400 C., indicating no phase transformation occuring over the entire range of temperature, 0 to l,400 C. An X-ray diffraction pattern taken from the specimen indicated that only the face-centered cubic austenitic phase was present.

Precipitation Hardening As previously mentioned, the maximum high-temperature strength of the alloy system within the scope of this invention can be developed if a suitable heat treatment is applied to the alloy in order to induce precipitation of a molybdenum-containing phase. This takes place in a two-step operation in, which the first step consists of a solid solutioning heat treatment or homogenization at a temperature in the range 1,000 to l,3l5 C. followed by a precipitation-inducing treatment and preferably at a temperature above the intended service design temperature. The solutioning step can be approached, in effect, by the use of a finish rolling temperature. The hightemperature solutioning heat treatment should be one which effects reasonably complete solutioning without causing excessive grain growth as determined by metallographic observation. The exact temperature or range of preferred temperature to achieve this effect for a given alloy is a function of the particular alloy composition and can be empirically determined by rnetallographic observation and hardness measurements. For an alloy containing 20 nickel, 5 percent chromium, and 10 percent molybdenum, a solutioning temperature of about 1,205 C. has been found to be satisfactory. The solutioned alloys are then aged at a temperature which will produce maximum and sustained hardness. Hardness (R curves for the Fe-ZONi-SCr-IOMo alloy composition are presented in FIG. I, which indicates the thermal treatment a.-Specimens were 0.076 cm. thick with a 0.63-cm.-wide reduced section and a 2.54-cm. gage length. The major axis of all specimens parallels the rolling direction.

b. MS-7O finish-rolled at 982 C.

c. All elevated-temperature testing of MS -70 FE-ZONi-SCr-IOMO was conducted in argon. I

d. The 316 SS was tested in airyield strengths are 0.2 percent offset values.

e. Yield strength value is based on deflectometer measurement of total load train elongation.

f. Composition of3l6 SS is 12.5 wlo Ni. 17.6 w/o Cr, 2.6 We M0, 1.9 w/o Mn.

Short Term Tensile Properties The purpose of this example is to demonstrate and compare the effects of a solution and aging heat treatment with the same alloy in the as-rolled and aged condition to determine the effect of the two processing methods on tensile strength and ductility. The results are summarized in Table 11. For comparison purposes, tensile properties of a 3 l 6 stainless steel are included as representative of an alloy known to be useful over the same range of temperature.

it will be noted that the solution and age-hardening treatment have apparently resulted in an increase in ductility accompanied by slight decreases in yield strength and tensile strength at all temperatures from room temperature to 750 C. However, from an overall standpoint, the alloy demonstrates, with or without a precipitation heat treatment, a useful combination of tensile strength and ductility and higher tensile strength than the comparison 316 stainless steel.

Creep This example is intended to illustrate the effect of prior processing heat treatment history on the creep rupture properties of the Fe-20Ni5Cr-10Mo alloy. In particular, it is designed to illustrate the effect of solutioning and precipitation-hardening on the creep properties of the alloy.

Sheet stock of the alloy were finish-rolled to 0.076 centimeter at two different temperatures, 750 and 982 C. Prior to the finish-rolling operation, one portion of sheet stock was aged at 750 C. for 20 hours while a second portion was solutioned at l,200 C. for 1 hour, water quenched to room temperature, and then aged with 750 C. Both portions were finish-rolled at either 760 or 982 C. and the results are summarized in Table III below.

OF THE Fe-20Ni-5Cr-10M0 ALLOY Finish Initial rolling exten- Linear b Rupture Rupture temp, sion, creep rate, lite, elong., C. Heat treatment percent in./in./hr. hr. percent At a Test Load of 1,960 kgJcm. at 650 0.

750 C.-20 hours 1. 10 2.4)(10- 11.6 43. 9 .do 0. 80 4X10' 35. 2 41. 4 1.00 2.6)(10' 66. 8 37. 5 1. 08 3X10- 68. 2 38. 1 2 o 1. 09 2.5X10" 73. 3 28. 760 1,200 C.-1 hours- 0. 98 1.1X10' 359. 5 3. 6

WQ 0 plus 750 C.-20

5.8X- 238. 3 21. 5 6.5)(10- 476 3. 4 5.3X10' 288. 2 21.1 315x10" 463. 6 2A. 7 1.5)(10' 148. 2 43. 3

1,050 kg/em. at 750 C.

11 760 750 C. hours 0.55 5.0X10' 5.5 57.2 12... 760 1,200 C.1 hour-WQ s 0.83 1.3X10" T 596 1.7

plus 750 C.20 hours. 13 982 do 0.70 27x10" 295.1 16.4 31685 Annealed at 1,120 0.. 0. 54 3.2X10- 87. 4 39. 5

T. No failure-test terminated.

at. Specimens were 0.076 cm. thick with a 0.63-cm-wide reduced section and a 2.54-cm. gage length. The major axis of each specimen parallels the rolling direction. All tests on developmental alloys conducted in argon.

b. Extension based on dial gage reading of total train elongation and is approximate. Values for Type 3 l 6 stainless steel were read directly from gage section.

c. WO water quench.

The results show the dramatic improvement in rupture life obtained with a solid solution-precipitation heat treatment over the same alloy in the as-rolled-and-aged condition without the solutioning-precipitation heat treatment. A comparison of the rupture life of the solutioned and precipitationheat-treated alloy with a 316 stainless steel shows similar significant improvements.

Creep rupture studies with lower amounts of molybdenum that we conducted showed that the same increased improvements cannot be obtained at molybdenum concentrations lower than about 5 percent.

Sheet specimens of the various heat-treated Fe-20Ni-5CrlOMo alloys were examined by electron microscopy techniques. Micrographs revealed coarse p.-phase (Fe-,Mo particles present in the as-rolled condition at 982 C. which appeared to coalesce and dissolved when solutioned at l,205 C. for l hour. A l,3l5 C. solutioning dissolves essentially all particles but causes considerable grain growth. Aging each of the solutioned microstructures at 750 C. allows the Fe Mo to precipitate as crystallographically oriented platelets. The development of the p-phase platelets is believed to be primarily responsible for the higher creep rupture strength. The improvement in creep of the Fe-2ONi-5Cr-1OM0 alloy can be illustrated in a somewhat different way by reference to FIG. 2, which is a Larson-Miller parametric representation of the Fe- ZONi-SCr-lOMo alloy compared with a 316 stainless steel. In the equation, on the abscissa, T is temperature in F., t is time in hours, and P is the parameter numbcr-a dimension less constant. The curve clearly shows the strength advantage of the Fe-ZONi-SCr-IOMo alloy over a considerable parametric range.

ln recapitulation, it has been shown that a selected class of iron-base alloys containing a specifically defined range of concentration of chromium, nickel, and molybdenum subjected to a solutioning and precipitation-inducing heat treatment operation provides an alloy system which is completely austenitic throughout the range of temperature from 0 to l,400 C. In addition, this class of alloys has significant shortand long-term strength as well as ductility for use at service temperatures up to as high as 750 C. in nonoxidizing atmospheres.

For the sake of clarity, the concentration of the claimed alloy of this invention is given below in terms of weight percent as well as atom percent.

As an Fe-Ni-Cr-Mo alloy, the invention consists essentially of in atom percent in weight percent Fe 50-7 l 46.29-68.56

Mo 4-l0 6.644590 As an Fe-Ni-Cr-W alloy, the invention consists essentially of in atom percent in weight percent Fe 50-71 40.39-64.64 Ni 20-30 19.14-25.48 Cr 5-l0 4.24-7.52 W 4-l 0 "SB-26.61

What is claimed is:

1. An iron-base alloy characterized in that (a) it consists of an iron-nickel-chromium matrix which maintains substantially complete austenitic stability at temperatures from 0 to l,400 C.; (b) it is susceptible to treatment to effect a precipitationhardening resulting in a molybdenum-containing precipitated phase; (0) said alloy consisting essentially of, in weight percent, an iron-base alloy containing 20.30 to 29.19 percent nickel, 4.50 to 8.62 percent chromium, 6.64 to 15.90 percent molybdenum, and 46.29 to 68.56 percent iron.

2. An iron-base alloy characterized in that (a) it consists of an iron-nickel-chromium matrix which maintains substantially complete austenitic stability at temperatures from 0 to 1.400" C.; (b) it is susceptible to treatment to effect a precipitationhardening resulting in a tungsten-containing precipitated phase; (0) said alloy consisting of, in weight percent, 19. l 4 to 25.48 percent nickel, 4.24 to 7.52 percent chromium, l 1,98 to 26.61 percent tungsten, and 40.39 to 64.64 percent iron.

3. The alloy of claim 1 which consists essentially of, in weight percent, 19 to 20 percent nickel, 4 to 5 percent chromium, 15 to 16 percent molybdenum, and the balance iron. 

2. An iron-base alloy characterized in that (a) it consists of an iron-nickel-chromium matrix which maintains substantially complete austenitic stability at temperatures from 0* to 1,400* C.; (b) it is susceptible to treatment to effect a precipitation-hardening resulting in a tungsten-containing precipitated phase; (c) said alloy consisting of, in weight percent, 19.14 to 25.48 percent nickel, 4.24 to 7.52 percent chromium, 11.98 to 26.61 percent tungsten, and 40.39 to 64.64 percent iron.
 3. The alloy of claim 1 which consists essentially of, in weight percent, 19 to 20 percent nickel, 4 to 5 percent chromium, 15 to 16 percent molybdenum, and the balance iron. 